Regional cerebral blood flow (rCBF) and quantitative EEG were studied in 15 patients with dementia of Alzheimer type (DAT), 15 patients with vascular dementia (VD) and 5 age-matched controls. The rCBF were determined by using SPECT with IMP as a tracer and ARG method (one point venous blood sampling). In the same patients, wave percent time in each frequency band of the EEG were measured using the wave-form recognition method. The DAT and VD patients were divided into 3 stages according to the severity of dementia assssed by the Clinical Dementia Rating (CDR). The rCBF and EEG variables (wave percent time) in the different stages were compared each other.In the severe DAT,the rCBF decreased significantly in the parietal area. In the severe VD,the rCBF decreased in the frontal area and basal gray region.In the mild DAT and VD,the percent time alpha-3 (11.5-13Hz) decreased significantly in the frontal and/or temporal areas. In the moderate DAT and VD,the percent time in almost all frequency bands changed significantly in the frontal, central and occipital areas.In DAT,rCBF in all areas did not significantly correlate with the percent time in each frequency. In VD,rCBF in left frontal area significanlty correlated with the percent time theta 2 (4-6Hz) and rCBF in right frontal area significantly correlated with the percent time delta (0.5-4Hz).